How do I address a question where the description does not match the title? For example, the question's title asks for one specific thing, and in the description, a different (sometimes multiple) questions are asked in addition.
Or, say that a question has multiple parts to it - If I have answers to part of the question - but not all of it - should I answer the question at all? Should I only answer if my answer covers the question in the title?
Generally these are signs of a problem with the question and indicate that it's not ready to be answered yet.
When a question contains multiple questions that can each be answered independently, they are probably separate questions that should be posted as separate question posts.
The thing to do here is leave a comment explaining that we need one question per post (so that each page can focus on fully answering just one question), and asking them to post their other questions separately. Then either edit the question to contain only one question by cutting out the rest, or (if that's not feasible) flag or vote the question closed as “too broad” and let the author sort it out.
When the title and body of a question really mismatch, something is missing that's making what the actual question is unclear. Maybe they have two questions (as above), maybe they're the same question but they left out some of the description that would connect the title and body together.
In general, the title is secondary because it's the shortest and can contain the least information, so if you're going to answer one or the other, it's the body that should be answered. If it's so bad that you can't tell how the title is related to the question body, then the question has enough problems that it would be better to hold off answering until it's fixed.
The thing to do here is flag or vote to close as “unclear” and (optionally) leave a comment asking for specific details that you think would clear it up.
In general, trying to answer a question that's a jumble of different questions or ideas tends to just make for more work, because after the question is fixed the answers usually need complete overhauls to become relevant to the fixed question.
You should vote to put the question on hold and work with the OP to refine their question in both cases.
